1. Field of the Invention
The present invention relates to a hollow formwork for use in making a concrete elongated member used to form a concrete roadbed of, for example, a high level roadway or a road bridge. Specifically, the concrete beam herein referred to may be such as used as a prestressed concrete bridge beam of a kind stipulated in JIS-A-5373 as a precast prestressed concrete product.
2. Description of the Prior Art
A concrete roadbed 100, shown in FIG. 12, of a high level roadway or a road bridge which is an example of the concrete product and is formed by a road subgrade 102 made up of a plurality of concrete beams 112 juxtaposed with each other in a direction widthwise of the roadway and laid above spaced piers 101. To complete the high level roadway or road bridge, concrete mortar is cast on an upper surface of the concrete roadbed 100. The concrete beams 112 are in the form of an elongated hollow concrete bar having an axially extending hollow 112a, as shown in FIG. 13, for the purpose of reducing the weight and increasing the strength. The hollow 112a in each of the concrete beams 112 is required to have a generally heptagonal shape according to JIS-A-5373 and, accordingly, in order to define this hollow 112a in each concrete beam 112, an inner hollow formwork 113 is embedded in the respective concrete beam 112.
As a material for the inner hollow formwork 113, a lightweight synthetic resinous foam material has hitherto been used. See, for example, the Japanese Laid-open Patent Publication No. 7-331614. It is, however, been found that where the roadbed 100 is formed with the concrete beams 112 each having embedded therein an inner hollow formwork 113 made of foamed styrene, a substantial amount of obnoxious or harmful industrial wastes would result in when the concrete roadbed is wrecked or dismounted in the future.
In view of the above, as a material for the hollow inner formwork 113, environmentally friendly wooden boards are employed. Also, for further reduction in weight, the use of hollow inner formworks has come to be contemplated, which is made of corrugated cardboard formed by bonding liners to opposite surfaces of a corrugated fiberboard. See, for example, the Japanese Utility Model Publication No. 60-2897.
Where the hollow inner formworks made of the corrugated cardboard are used in forming the concrete beams 112, each hollow inner formwork 113A is, as shown in FIG. 14, placed inside an elongated outer mold 114 and, then, ready-mix concrete is poured into the elongated outer mold 114 so as to encompass the hollow inner formwork 113A. However, it has been found that, during the concrete casting, portions of the corrugated cardboard 120 forming the hollow inner formwork 113A tend to be collapsed as shown in FIG. 15 under the influence of a compressive force brought about by the ready-mix concrete being externally poured. Once the hollow inner formwork 113A deforms as a result of the collapse of the corrugated cardboard 120, the amount of concrete 108 cast at collapsed portions 120 of the corrugated cardboard 120 will increase with the balance of the strength of the respective concrete beam 112 ruined consequently, resulting in reduction of the overall strength of the concrete beam 112. Accordingly, the hollow inner formwork 113A made of corrugated cardboard, which is actually used, is reinforced with a reinforcement material such as, for example, wooden board. The additional use of the wooden board to reinforce the hollow inner formwork 113A made of corrugated cardboard results in increase of the weight and, hence, increase of the cost as compared with the sole use of the corrugated cardboard.